Visitors to Simon Halabi's office in Mayfair are allowed a rare treat. On a table laid with a map of London, models of the buildings he owns are placed in their real-life locations, like a Monopoly board. This impressive diorama, taking in landmarks such as the Aviva Tower and Millennium Bridge House, is a statement of wealth made by a man whose fortune has been counted in the billions.

Not any more, it seems. Despite his importance as a London landlord, underlined by the display in his office, Halabi's riches have been called into question. He was recently dropped from the Estates Gazette Rich List of property tycoons and, the Evening Standard has been told, he will not feature in the forthcoming Sunday Times compilation of Britain's wealthiest individuals.

This year he was ranked 31st, with an estimated fortune of £2 billion. But concerns over reverses in his business affairs and difficulties in gauging how much he is worth now mean that, as things stand, he will be dropped from the 2009 list.

No one is suggesting Halabi is broke. Far from it. The Syrian-born magnate, who is in his fifties, still has a house off Berkeley Square, a château in France, a superyacht and a fleet of expensive cars. He also has his London buildings and others elsewhere but property world sources say he has been badly affected by the economic downturn and the vast empire over which he once presided may not survive intact.

At a time of disintegrating fortunes and dwindling prospects almost everywhere, this might raise only marginal concern outside the real estate business. But Halabi's portfolio includes two buildings many would think of as national treasures, the Naval and Military Club on Piccadilly and Mentmore Towers, the former Rothschild home in Buckinghamshire.

One of Halabi's companies acquired Mentmore, as it is more generally known, in 1997. At that time it was badly in need of restoration. Finished in 1854 for Baron Mayer de Rothschild, it was designed by Joseph Paxton, architect of the Crystal Palace. The house was once the home of former prime minister Lord Rosebery but when Halabi bought it the owners were the Maharishi Yogi's transcendental meditation movement. Mentmore had suffered considerable wear and tear. One of its main uses latterly had been as a film set and it was once used as Bruce Wayne's mansion in a Batman movie.

Halabi had great plans for Mentmore. It was to become Britain's first six-star hotel, its magnificent interiors returned to their Victorian splendour, with 100 luxurious suites. An early obstacle over planning permission was overcome and residents in the nearby village of Mentmore waited for the builders to move in. They are still waiting.

Today, Mentmore, Grade II* listed, is a building in need of help. English Heritage has placed it on the At Risk register and a spokeswoman told the Standard it needs urgent work on the roof and chimneys. There is concern that weather will penetrate to the interiors, considered among the finest examples of Victorian design and craftsmanship in Britain. "It has planning consent but no work is being done," the spokeswoman said. "The project seems to have stalled."

A Mentmore village resident said that apart from the felling of some trees about 18 months ago there had been no sign of activity at the house. "It's very sad," the villager said. "It's just standing empty and you wonder whether anything will ever happen."

Officials in Westminster have been thinking the same thing about the former Naval and Military Club on Piccadilly. This Grade I listed building is known affectionately as the In and Out Club because of the prominent signs on the entrance and exit. The club moved to new premises in 1996 and sold the house to Halabi's company.

Among London houses, the late Palladian mansion is considered a gem. It is the former home of Lord Palmerston, one of Queen Victoria's most celebrated prime ministers, and its location at 94 Piccadilly, overlooking Green Park and Buckingham Palace, gives it a very special status.

Halabi wanted to turn it into a luxury hotel that would rank with anything in the world. He has planning permission to do precisely that but nothing seems to be happening, apart from a bit of demolition at the rear.

The only use to which Palmerston's old home is currently being put is as a handy place to park Halabi's cars. When he is in his nearby office, his Range Rover, or his black Hummer, with their distinctive BSH number plates (his first name, which he rarely uses, is Bassam), are often parked in the In and Out driveway. Since Halabi paid £50 million for the house, it is probably London's most expensive parking spot.

Inevitably, perhaps, lack of progress on his plans for Mentmore and 94 Piccadilly has led to speculation that Halabi no longer has the resources to follow through his ideas. This would cause some anxiety in heritage circles, if not elsewhere. But Halabi is a man who carefully shades his hand from other players and no one really knows what his intentions are. Indeed, he has a reputation for being secretive. At one time only one photograph of him had ever been seen outside his closest circle. It was a small head-and-shoulders shot that was given to a business magazine some years ago. When it was used more widely by a newspaper he was infuriated and complained.

Halabi's wealth derived initially from his family. His father had extensive industrial and agricultural holdings in Syria and when the teenage Simon moved to London to start out on his own, he had considerable backing. He met his wife, Urte, a Lithuanian, in London and the couple have a son, Jacob.

Four years ago they were struck by tragedy. The couple were on holiday at their château near Grasse in the south of France. They had not visited the property for some time and were busy clearing out a pond. As they worked their two-year-old son, Samuel, walked away unnoticed and fell into the swimming pool. By the time they realised what had happened it was too late. The child drowned.

Halabi is the most intensely private of men and has never spoken about this personal catastrophe but it has been said he is still haunted by the loss of his son and the August anniversary of the child's death still inflicts pain on him and his family. The tragedy was followed by a setback over one of Halabi's most ambitious projects.

He had a stake in the Shard of Glass, the proposed 1,016ft tall building planned for a site at London Bridge station. The Shard was billed as Europe's tallest structure, with offices, apartments and a super-luxury hotel. From its upper floors, it was boasted, you would be able to see the sea.

But Halabi fell out with his partners in the project, notably Irvine Sellar the 1970s Carnaby Street fashion king turned property developer. The dispute became increasingly bitter and the two tycoons ended up in court. The details of their settlement are not known but it was understood Halabi sold out his interest in the Shard to Qatari investors.

The Shard is yet to be built and if Halabi was disappointed by what happened, he has never admitted it. But there can be no doubt about his regrets over another big deal that came afterwards. Early last year Halabi's companies departed from property investment and bought Esporta, one of Britain's biggest health club chains.

Competition for the business was lively. Esporta had prospered in the fitness boom and the price was based on a heady multiple of 13 times earnings. It meant prospective buyers had to come up with £475 million. Halabi did just that but it was not all his money. He is thought to have put in £115 million of his own cash and secured a £30 million loan from the private equity group that owned Esporta.

The majority of the purchase price, £330 million, was provided by Société Gé*érale, the French bank which had been backing Halabi for years. Almost as soon as the deal was done, problems arose. Esporta's earnings slumped and one valuation suggested the business was worth no more than the amount owed to SocGen. Property circle sources said Halabi was facing a loss of more than £140 million and Esporta's holding company, whose ultimate owners are Halabi's family trusts based in Jersey, was placed in administration.

That was last August and shortly afterwards Halabi took his family on a cruise in the Mediterranean aboard their 130ft yacht, Samja. His apparent insouciance was noted and some observers concluded that since his property portfolio was valued at around £5 billion, he was not unduly worried.

Then, as property values went into freefall, those same observers began wondering how much of his wealth comprised equity and how much represented leverage. At that time it was generally believed Halabi's personal wealth was around £2 billion - but was it?

The figures are not available for public scrutiny but it was noted that he had considered scaling down his London assets with possible sales or even a flotation. He was also said to have thought of bringing in outside investors with a bond issue. So far, nothing seems to have come of these initiatives.

The Evening Standard tried to contact Halabi to discuss these issues, without success. A request for information met with no response. Meanwhile, weeds are growing through the cracks in the driveway at the old In and Out and English Heritage is fretting over the future of Mentmore. Perhaps it is time, now, for Halabi to begin moving the pieces on his reality Monopoly board.